Crystal grain size reduction method for plating film

ABSTRACT

In a crystal grain size reduction method for a plating film, electroplating is performed in a condition where ions of a plating metal, a nanocarbon, and an anion based surfactant as a dispersion agent for dispersing the nanocarbon have been blended in a plating solution.

TECHNICAL FIELD

The present invention relates to a method for forming reduced-sizecrystal grains of a plating film.

BACKGROUND Related Art

Composite plating in which small particles have been eutecticallydeposited in a plating metal film has been known. For example, JapanesePatent Application Publication No. 2008-214667 mentions azinc-nanocarbon composite plating. In the above composite plating, azinc plating film is formed on a plating object by using a zinc platingsolution into which nanocarbon, and polyacrylamide as a dispersion agentfor the nanocarbon, have been added.

Japanese Patent Application Publication No. 2008-214667 also mentionsthat it is preferable that nanocarbon be present in the zinc platingfilm and that the amount of nanocarbon added into the zinc platingsolution be 0.5 to 5.0 g/L. Furthermore, Japanese Patent ApplicationPublication No. 2008-214667 indicates that because part of thenanocarbon is exposed out of the zinc plating film, a zinc plating filmthat exhibits excellent sliding characteristics can be made.

It is generally considered that, as in the technology described inJapanese Patent Application Publication No. 2008-214667, incorporationof nanocarbon into a plating film will reform the surface of the platingfilm. For example, the incorporation of nanocarbon into the plating filmis considered to harden the plating film and improve anti-abrasionproperties associated with sliding.

Actually, however, it is not that the plating film is made hard but thatthe nanocarbon particles in a surface layer are hard. Thus, theanti-abrasion property of the plating film is not a simple property thatdepends only on the hardness of the plating film, but may be affected bya combination of various other elements such as the surface roughness(sliding property), the lubricity of the plating, and the toughness andcrystal grain size of plating metal.

In practice, even in the case of a plating metal having high hardnessand good sliding property (a plating metal having small crystal grains),because of the hard plating surface, once sliding results in a platingsurface (contact surface) having a chip (flaw) formed by galling or thelike, the flaw sharply increases the friction coefficient of the platingsurface. As a result, the plating surface is further damaged andabrasion rapidly progresses. The above phenomenon is more likely tooccur on plating metals that have high hardness but low toughness (e.g.plating metals having brittle grain boundaries and weak binding force).On the other hand, in the case of a plating metal having relatively lowhardness, chipping does not occur but its low hardness makes the wearrate high and cannot bring about high anti-abrasion property.

Therefore, it cannot generally be said that incorporation of nanocarboninto a plating film will reform the surface of the plating film.Furthermore, in the case where nanocarbon is incorporated into a platingfilm, it is very difficult to uniformly disperse the nanocarbon in theplating film or precisely control the amount of the nanocarbon containedin the plating film. In addition, since nanocarbon is a nonconductor,use of a nanocarbon-incorporated plating film on an electrical contactpoint makes the electrical contact resistance unstable and also greatlyincreases electrical contact resistance.

SUMMARY

In view of the problems outlined above, it is an object of the presentinvention to provide a crystal grain size reduction method for a platingfilm which is capable of reforming the surface of the plating filmwithout substantial incorporation of nanocarbon into the plating film.

As a result of vigorous study to solve the aforementioned problems, thepresent inventor found that crystal grains of a plating film can bereduced in size by causing nanocarbon to function as if it was acatalyst, while avoiding incorporation of nanocarbon into the platingfilm, thereby realizing some aspects associated with the presentinvention. Specifically, to solve the foregoing problems, arepresentative construction of a crystal grain size reduction method fora plating film according to the present invention is characterized byperforming electroplating in a condition where ions of a plating metal,a nanocarbon, and an anion based surfactant as a dispersion agent fordispersing the nanocarbon have been blended in a plating solution.

According to the above-described construction, since the dispersionagent is blended in the plating solution, the nanocarbon is dispersed inthe plating solution with molecules of the dispersion agent adsorbed tothe nanocarbon. Due to the use of an anion based surfactant as adispersion agent, the nanocarbon dispersed in the plating solution isnot readily incorporated into the surfaces of parts to be plated(plating objects) that are connected to the negative electrode. On thesurfaces of the plating objects, epitaxial growth of the plating metalproceeds to form crystal grains. The nanocarbon affects the epitaxialgrowth of the plating metal so as to reduce the size of the crystalgrains of the plating film. Although the behavior during this process isnot entirely known, it can be hypothesized that, due to the Brownianmotion in the plating solution, nanocarbon particles come into contactwith and exert forces to crystal grains, thereby reducing the size ofthe crystal grains. Thus, the present invention realizes reform of thesurface of a plating film by reducing the size of crystal grains of theplating film without substantial incorporation of nanocarbon into theplating film.

It is appropriate that the nanocarbon be positively charged when in astate of mixture with the plating solution. It is hypothesized that thepositively charged nanocarbon in the plating solution (despite moleculesof the anion based surfactant being adsorbed to nanocarbon particles) isattracted to the surface of the part to be plated that is connected tothe negative electrode. Due to attraction to the surface of the part tobe plated, the nanocarbon particles can certainly come into contact withand exert forces to crystal grains of the plating film, reliablyreducing the size of the crystal grains of the plating film.

It is appropriate that the particle diameter of the nanocarbon be2.6±0.5 nm. With the particle diameter of the nanocarbon being in thisrange, the nanocarbon particles in the plating solution certainlyundergo Brownian motion and, when contacting crystal grains of theplating film, exert on the crystal grains appropriate forces thatreduces the size of the crystal grains. It is hypothesized that the sizereduction of the crystal grains is insufficient when the particlediameter of the nanocarbon is above the aforementioned range because theBrownian motion of the nanocarbon particles is not sufficient andtherefore cannot exert appropriate forces on the crystal grains.Further, it is also hypothesized that the crystal grain size reductionbecomes insufficient when the nanocarbon particle diameter is below theabove range because, despite occurrence of Brownian motion, the smallmasses of the nanocarbon particles cannot exert to the crystal grains aforce sufficient for size reduction of the crystal grains.

It is appropriate that the amount of the nanocarbon added into theplating solution be less than or equal to 0.2 g/L. By setting the amountof the nanocarbon added to a small amount that is less than or equal to0.2 g/L, the nanocarbon can be prevented from being incorporated intothe plating film.

It is appropriate that the plating metal be silver (Ag), nickel (Ni),tin (Sn), or gold (Au). Accordingly, a plating solution that is neutralor weakly acidic may be used.

According to the present invention, it is possible to provide a crystalgrain size reduction method for a plating film which is capable ofreforming a surface of a plating film without substantial incorporationof nanocarbon into the plating film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that generally illustrates a crystal grain sizereduction method for a plating film according to an exemplary embodimentof the invention.

FIGS. 2A and 2B show microscopic photographs that exhibit a plating filmobtained as illustrated in FIG. 1 and a plating film of a comparativeexample, respectively.

FIGS. 3A and 3B are schematic diagrams of the plating films exhibited inFIGS. 2A and 2B, respectively.

FIGS. 4A and 4B are graphs representing the durabilities and the contactresistances of the plating films exhibited in FIGS. 2A and 2B,respectively.

FIGS. 5A and 5B show microscopic photographs that exhibit plating filmsaccording to another exemplary embodiment of the invention and anothercomparative example.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will be described indetail hereinafter with reference to the accompanying drawings. Thedimensions, materials, and other concrete numerical values mentioned inconjunction with the following exemplary embodiments are merelyillustrative for the sake of easy understanding of the invention and notintended to limit the scope of the invention unless otherwise indicated.In the description and drawings, elements substantially the same infunction and construction are indicated by the same reference charactersand are not redundantly described. Furthermore, elements and the likethat are not directly relevant to the present invention are omitted fromgraphical representation.

FIG. 1 illustrates a crystal grain size reduction method for a platingfilm according to an exemplary embodiment. The size reduction method ofthis exemplary embodiment may be carried out, for example, by using aplating apparatus 100. The plating apparatus 100 is an apparatus forcarrying out electroplating and includes a container 102, a platingsolution 104 in the container 102, a negative electrode 106 and apositive electrode 108 immersed in the plating solution 104, and anelectricity source 110 that applies voltage between the two electrodes.

Blended in the plating solution 104 are ions of a plating metal 112, ananocarbon 114, and a dispersion agent 116. The plating metal 112, asshown in this example is a monovalent cation of silver (Ag).

The dispersion agent 116 used in this example is an anion basedsurfactant. As illustrated in FIG. 1, when molecules of the surfactantare adsorbed to a nanocarbon particle 114, the liphophilic group 118 bof each surfactant molecule becomes attached to the nanocarbon particle114, with the hydrophilic group 118 a of each surfactant moleculepositioned outward. Therefore, the nanocarbon particles 114 do notaggregate but are dispersed in the plating solution 104 due to thedispersion agent 116.

As for the nanocarbon 114, for example, the amount added to the platingsolution 104 was set to 0.2 g/L, and the particle diameter of thenanocarbon 114 was set to 2.6±0.5 nm. Furthermore, the nanocarbonparticles 114 in a mixture with the plating solution 104 are positivelycharged. The plating solution 104 is neutral because the plating metal112 is silver (Ag).

When a plating process is started in the plating apparatus 100 byapplying, from the electricity source 110, a voltage between thenegative electrode 106 and the positive electrode 108, then epitaxialgrowth of the plating metal 112 progresses on the surface of a platingobject 120 connected to the negative electrode 106, so that crystalgrains of the plating metal 112 form. As a result, the surface of theplating object 120 has on its surface a plating film 122 as indicated byhatching in FIG. 1.

FIGS. 2A and 2B show microscopic photographs of a plating film 122formed as illustrated in FIG. 1 and a plating film 122A formed as acomparative example. The plating film 122 shown in FIG. 2A was obtainedby adding the nanocarbon 114 into the plating solution 104 according tothe crystal grain size reduction method of the exemplary embodiment. Theplating film 122A of the comparative example shown in FIG. 2B wasobtained without adding the nanocarbon 114 into the plating solution104.

Observation of the microscopic photographs of the plating films 122 and122A reveals that the crystal grains of the plating film 122 are clearlysmaller in size than the crystal grains of the plating film 122A.Therefore, it is clear that the crystal grain size reduction method ofthis exemplary embodiment is capable of reducing the size of the crystalgrains (forming nanocrystal grains) of the plating film 122. Table 1,presented below, compares the carbon contents of the plating films 122and 122A.

TABLE 1 Carbon content of plating Addition of nanocarbon film (mass %)No 0.00182 Yes 0.00178

As depicted in Table 1, the carbon content of the plating film 122according to this exemplary embodiment in which the nanocarbon 114 wasadded was substantially the same as the carbon content of the platingfilm 122A of the comparative example in which the nanocarbon 114 was notadded. Thus, it is clear that the plating film 122 formed by the crystalgrain size reduction method of the exemplary embodiment did notsubstantially incorporate the nanocarbon 114.

Thus, in the crystal grain size reduction method according to theexemplary embodiment, size reduction of the crystal grains of theplating film 122 is achieved by the nanocarbon 114 functioning as if itwas a catalyst, without substantial incorporation of the nanocarbon intothe plating film 122. This phenomenon will be discussed below.

First of all, the nanocarbon 114 dispersed in the plating solution 104is not readily incorporated into the plating film 122 on the surface ofthe plating object 120 that is connected to the negative electrode 106because an anion based surfactant is used as the dispersion agent 116.In addition, since the amount of the nanocarbon 114 added is as small as0.2 g/L, incorporation of the nanocarbon 114 into the plating film 122does not easily occur in the first place. Thus, in the conditions asindicated above, the nanocarbon 114 was, actually, negligiblyincorporated into the plating film 122.

Next, it is hypothesized that, because the nanocarbon particles 114 arepositively charged in the plating solution 104, the molecules of theanion based surfactant adsorbed to the nanocarbon particles 114 do notprevent the nanocarbon particles 114 from being attracted to the surfaceof the plating object 120 connected to the negative electrode 106, sothat the nanocarbon particles 114 can affect the epitaxial growth of theplating metal 112.

Although the behavior of the nanocarbon particles 114 during thisprocess is not entirely known, it can be hypothesized that, due to theBrownian motion in the plating solution 104, the nanocarbon particles114 come into contact with and exert forces to crystal grains, therebyachieving size reduction of the crystal grains. Specifically, it can behypothesized that the positively charged nanocarbon 114 in the platingsolution 104 is attracted to the surface of the plating object 120thereby definitely coming into contact with, and exerting forces on,crystal grains of the plating film, so that the crystal grains of theplating film can be definitively reduced in size.

Furthermore, since the particle diameter of the nanocarbon 114 is setwithin the range of 2.6±0.5 nm, the particles of the nanocarbon 114 inthe plating solution 104 certainly undergo Brownian motion, so that asnanocarbon particles 114 come into contact with crystal grains of theplating film, the nanocarbon particles 114 exert to the crystal grainsappropriate forces that reduce the size of the crystal grains. It ishypothesized that size reduction of the crystal grains becomesinsufficient if the particle diameter of the nanocarbon 114 is above theaforementioned range because the Brownian motion of the nanocarbonparticles is not sufficient and therefore cannot exert appropriateforces on the crystal grains. On the other hand, it is furtherhypothesized that crystal grain size reduction becomes insufficient whenthe particle diameter of the nanocarbon 114 is below the range isspeculated so that, despite occurrence of the Brownian motion, the smallmasses of the nanocarbon particles cannot exert, on the crystal grains,a force sufficient to reduce the size of the crystal grains.

Plating object 120 may be provided with plating film 122 that can beused as an electrical contact. Therefore, it is desirable for theplating film 122 to have a low electric resistivity (contactresistance). Furthermore, since the plating object 120 may be repeatedlyinserted into a socket or the like, it is also desirable that theplating film 122 have high durability (i.e., greater anti-abrasionproperties that may be associated with sliding).

A crystal structure of a metal will be described with reference to FIGS.3A and 3B. FIGS. 3A and 3B are schematic diagrams that correspond to theplating films 122 and 122A shown in FIGS. 2A and 2B.

The metal can be viewed as a crystal grain aggregate which includescrystal grains and grain boundaries that surround the crystal grains(defects of crystals or impurities) and in which crystal grains arebound to each other at grain boundaries. The abrasion of metal caused bysliding occurs in two different ways: crystal grains themselves undergotransgranular fracture; and boundaries fracture and the metal chips anderodes in block units of crystal grains. In this exemplary embodiment,it is an object to restrain the grain boundary fracture in which themetal chips off grain by grain and therefore increase the durability. Inmetal, when grain boundary fracture involves erosion of large crystalgrains, the chipped-off volume, that is, the amount of erosion, islarge; on the other hand, erosion of a small crystal grain means a smallamount of erosion. Furthermore, in metal, since crystal grains are boundtogether at grain boundaries, it is hypothesized that the greater thestrength of grain boundaries and the binding force thereof, the lesseasily erosion occurs. Therefore, crystal structure features of a metalthat facilitate realization of a highly durable plating film are thatthe crystal grains are small and that the binding force at grainboundaries that binds crystal grains together is strong.

In the plating film 122 illustrated in FIG. 3A, the crystal grains 124are smaller than the crystal grains 124A of the plating film 122Aillustrated in FIG. 3B and the grain boundaries 126 where crystal grains124 are bound together outnumber the grain boundaries 126A of theplating film 122A. Therefore, the plating film 122 is less easilyerodable to sliding and therefore more durable than the plating film122A.

Moreover, it is generally considered that metal becomes harder as thecrystal grains are made smaller. In this regard, the plating film 122Aof the comparative example, in which the crystal grains 124A were notreduced in size, had a Vickers hardness of 90 to 110 Hv. On the otherhand, the plating film 122 of the exemplary embodiment, in which thecrystal grains 124 were reduced in size, had a Vickers hardness of 100to 110 Hv, making it clear that size reduction of the crystal grains 124did not make the plating film 122 harder. Therefore, good running-inproperty (lubricity) of the contact surface (which is a characteristicof silver (Ag)) of the plating metal 112 can be maintained, so that thesurface of the plating film 122 (the contact surface at the time ofsliding) is smooth and the friction coefficient does not considerablychange even after repeated sliding. Thus, the durability of the platingfilm 122 can be increased.

Next, the electrical contact resistance of the plating film 122 will bedescribed. It is considered that when the crystal grains of metal aremade smaller, grain boundaries generally increase, so that theelectrical contact resistance increases. However, in the plating film122 according to the exemplary embodiment, although the crystal grains124 are small, the contact resistance is not high but about 3×10⁻⁶ toabout 3.5×10⁻⁶ Ω·cm. Incidentally, the contact resistance of a superhard silver plating that has substantially the same crystal graindiameter is as high as greater than or equal to 8×10⁻⁶ Ω·cm. A reasonfor this is speculated to be that because the size reduction of thecrystal grains 124 of the plating film 122 does not involve the alloyingwith a different metal, such as antimony (Sb), or because no adsorbentorganic luster is used in the plating film 122, the grain boundaries 126contain only a small amount of impurities.

FIGS. 4A and 4B are graphs indicating the durability and the contactresistance of the plating films 122 and 122A illustrated in FIGS. 2A and2B, respectively. In each one of the graphs in FIGS. 4A and 4B, thehorizontal axis represents the number of back-and-forth movements(number of sliding movement cycles) and the vertical axis represents thefriction force (N) or the resistance value (me). Note that high frictionforces, meaning high friction coefficient, are considered to mean thatabrasion of the surface easily progresses and the anti-abrasion propertythereof, that is, the durability thereof, is low.

The plating film 122 described in FIG. 4A produces a smaller frictionforce as a whole than the plating film 122A described in FIG. 4B, andtherefore is higher in anti-abrasion property. In fact, the plating film122 remained unfractured even when the number of back-and-forcemovements reached 1000. On the other hand, the plating film 122A, beinglow in anti-abrasion property, was destroyed as exhibited in FIG. 4Bwhen the number of back-and-force movements was about 600. Furthermore,the plating film 122 exhibited stable electrical resistance at lowresistance values, compared with the plating film 122A. On the otherhand, the plating film 122A exhibited unstable electrical resistancevalues as a whole. Furthermore, the resistance value of the plating film122A rapidly increased as the plating film 122A was fractured at thetime of about 600 back-and-forth movements.

Thus, it was made clear that the plating film 122 formed by the crystalgrain size reduction method according to the exemplary embodiment waslower in contact resistance and higher in durability than the platingfilm 122A of the comparative example, in which nanocarbon 114 was notadded into the plating solution 104. That is, in the crystal grain sizereduction method according to the exemplary embodiment, reform of thesurface of the plating film 122 is realized by reducing the size of thecrystal grains of the plating film 122 without substantial incorporationof the nanocarbon 114 into the plating film 122 although the nanocarbon114 is added into the plating solution 104.

Examples and comparative examples in which different amounts of thenanocarbon 114 were added will be described below. Table 2 describesExamples 1 and 2 and Comparative Examples 1 and 2. In Examples 1 and 2,the amounts of the nanocarbon 114 added were 0.1 g/L and 0.2 g/L,respectively. In Comparative Example 1, the amount of the nanocarbon 114added was zero, that is, no nanocarbon 114 was added. In ComparativeExample 2, the amount of the nanocarbon 114 added was 0.3 g/L.

TABLE 2 The amount of Evaluations nanocarbon Size of Anti- added crystalabrasion Volume (g/L) grains property resistance Comparative 0 Large Nogood Low Example 1 and soft Example 1 0.1 Quite Acceptable Low smallExample 2 0.2 Quite Acceptable Low small Comparative 0.3 or IntermediateAcceptable High Example 2 larger

As mentioned in Table 2, in Comparative Example 1 with no nanocarbon 114added into the plating solution 104, the size of the crystal grains ofthe plating film was “Large”, the anti-abrasion property (durability)thereof was “No good”, and the volume resistance (electrical resistance)thereof was “Low”. In Comparative Example 2 with the amount of thenanocarbon 114 added being greater than or equal to 0.3 g/L, the size ofthe crystal grains of the plating film was “Intermediate”, theanti-abrasion property thereof was “Acceptable”, and the volumeresistance thereof was “High”. In contrast, in both Examples 1 and 2with the amount of the nanocarbon 114 added being less than or equal to0.2 g/L, the size of the crystal grains of the plating film was “Quitesmall”, the anti-abrasion property thereof was “Acceptable”, and thevolume resistance thereof was “Low”. Therefore, it is clear that if theamount of the nanocarbon 114 added is less than or equal to 0.2 g/L, thesize of the crystal grains of the plating film 122 can be made quitesmall and the anti-abrasion property thereof can be made high and,furthermore, the volume resistance thereof does not increase but remainslow despite the quite small size of the crystal grains.

FIGS. 5A and 5B show microscopic photographs exhibiting a plating film128 according to another exemplary embodiment and a plating film 128A ofanother comparative example. The plating film 128 according to theanother exemplary embodiment exhibited in FIG. 5A is different from theabove-described plating film 122 in that the plating metal 112 of theplating film 128 was nickel (Ni) instead of silver (Ag). The platingfilm 128A of the another comparative example exhibited in FIG. 5B wasobtained by using as the plating metal 112 nickel (Ni) instead of silver(Ag) and by omitting addition of the nanocarbon 114 into the platingsolution 104. Note that, due to the use of nickel (Ni) as a platingmetal, the plating solution 104 was weakly acidic.

Observation of the microscopic photographs of the plating films 128 and128A reveals that the crystal grains of the plating film 128 are clearlysmaller than the crystal grains of the plating film 128A. Therefore, itis clear that the crystal grain size reduction method according to theanother exemplary embodiment is able to reduce the size of the crystalgrains of the plating film 128. Table 3, presented below, indicatesresults of a sliding test of the plating films 128 and 128A.

TABLE 3 Number of slidings Load (g) Nickel sulfamate +Nanocarbon 50434.35 520.55 50 426.91 513.61 50 423.91 526.31 50 426.9 523.30 50423.88 529.15 50 416.68 526.33 Average 425.4 523.2

The plating film 128A of the comparative example was formed by blendingnickel sulfamate in the plating solution 104 and omitting addition ofthe nanocarbon 114 into the plating solution 104. As indicated in Table3, the plating film 128A was subjected to repeated sliding with a loadof 50 g and was destroyed when the number of sliding cycles reached,averagely, 425.4.

On the other hand, the plating film 128 according to the anotherexemplary embodiment was formed by blending nickel sulfamate in theplating solution 104 and adding the nanocarbon 114 into the platingsolution 104. As indicated in Table 3, the plating film 128 wasdestroyed when the number of sliding movements reached, averagely,523.2. This clarifies that the plating film 128 was more durable thanthe plating film 128A of the another comparative example.

Therefore, by the size reduction method according to this exemplaryembodiment, reform of the surfaces of the plating films 122 and 128 canbe realized by making the crystal grains of the plating films 122 and128 quite small without incorporation of the nanocarbon 114 into theplating films 122 and 128, respectively.

Incidentally, although in the foregoing exemplary embodiments of theinvention, the plating metal 112 is silver (Ag) or nickel (Ni) as anexample, these examples are non-limiting. The plating metal 112 may alsobe tin (Sn) or gold (Au). In such cases, it is hypothesized that thecrystal grains of the plating film can also be made quite small toreform the surface of the plating film by causing the nanocarbon 114 tofunction as if the nanocarbon 114 was a catalyst, while avoidingincorporation of the nanocarbon 114 into the plating film.

While the exemplary embodiments of the invention have been describedwith reference to the drawings, it should be apparent that the inventionis not limited by the foregoing examples or the like. It should beunderstood that a person having ordinary skill in the art can conceivevarious changes and modifications within the scope described in theappended claims and that such changes and modifications belong to thetechnical scope of the present invention.

The invention can be utilized as a method for forming reduced-sizecrystal grains of a plating film.

FIG. 4A PARAMETERS FRICTION FORCE (N)

RESISTANCE VALUE (mΩ)

NUMBER OF BACK-AND-FORTH MOVEMENT CYCLES FIG. 4B PARAMETERS FRICTIONFORCE (N)

RESISTANCE VALUE (mΩ)

NUMBER OF BACK-AND-FORTH MOVEMENT CYCLES

What is claimed is:
 1. A crystal grain size reduction method for aplating film comprising: performing electroplating in a condition whereions of a plating metal, a nanocarbon, and an anion based surfactant asa dispersion agent for dispersing the nanocarbon have been blended in aplating solution.
 2. The crystal grain size reduction method for theplating film according to claim 1, wherein the nanocarbon is positivelycharged when in the blended state with the plating solution.
 3. Thecrystal grain size reduction method for the plating film according toclaim 1, wherein the plating metal is one of silver, nickel, tin, orgold.
 4. The crystal grain size reduction method for the plating filmaccording to claim 2, wherein the plating metal is one of silver,nickel, tin, or gold.